In the analysis of samples for the determination of the quantity of radioactive constituents, it is conventional practice to place the sample proximate a gas-filled proportional counter. The radioactive decay products of the sample, either alpha particles or beta particles, or both, cause ionization of the gas giving rise to positive ions and electrons. The electrons are placed under the influence of an electric field created between an anode and the grounded surface of the detector. This results in a current at the anode that can be related to the radioactive constituent concentration within the sample.
A typical system for this type of analysis is that described in a brochure of Tennelec Model LB 5500 automatic low background alpha/beta counting system of Tennelec/Nucleus, Inc., Oak Ridge, Tennessee. This particular system utilizes a pair of "pancake"-type gas flow proportional counters stacked one above the other. The upper detector senses high energy cosmic radiation, while the lower detector analyzes for alpha and beta radiation emitted from a sample contained in a planchet placed below. Each of the detector units . contains at least one anode traversing the pancake, with the anode maintained at a potential suitable for collection of the electrons being produced in the detector gas by the radiation. The grounded surface of the pancake is typically known as the "puck". Typically, the pancake adjacent the sample planchet has a very thin window so as to contain the gas within the proportional counter.
Another Tennelec system utilizing this type of detector is described in a bulletin on Model LB 4000, a multi-detector low level alpha/beta counting system.
Although this system has been used effectively for radiation detection, there have been certain disadvantages to the detector design. One is the non-uniformity of the field strength (the potential difference between the anode and ground). This field strength was found to vary along the wire. This is due to the close spacing toward the ends of the anode wires, and then any spacing toward adjacent anode wires if more than one is used. These field differences are caused by the different spacing distances for a given applied voltage on the anode. Further, in a windowless configuration, if the sample develops an electrical charge, this charge adversely affects the field distribution.
Another detector construction for determining ionizing particle presence is described by 0. Bunemann, T. E.Cranshaw and J. A. Harvey in "Design of Grid Ionization Chambers" as published in Canadian Journal of Research, Vol. 27, Sec. A, pp. 191-206. This experimental gas-filled ion chamber (not a proportional chamber) consisted of two parallel plates mounted within a cylindrical vessel. Mounted in front of the collector was a grid of parallel wires to shield the collector from positive ions generated in the ionization process. The theory of that system, which is incorporated herein by reference, and some experimental results are reported in that publication.
It is an object of the present invention to provide a pancake-type radiation detector having at least one anode traversing across the pancake wherein substantially improved uniformity of field strength is achieved along the anode.
It is another object of the present invention to provide a pancake-type detector for ionizing particles having at least one anode traversing across the pancake, the anode being surrounded by a perforated grid spaced therefrom, whereby substantially improved uniformity of field strength is achieved along the anode.
It is a further object of the present invention is to provide a pancake-type detector for ionizing particles having at least one anode traversing across the pancake, the anode being surrounded by a perforated grid spaced therefrom, the grid being maintained at an intermediate potential between the potential of the anode and ground, whereby substantially improved uniformity of field strength is achieved along the anode.
Another object of the present invention is to provide a pancake-type detector for ionizing particles that does not require a window to admit the ionizing particles thereby increasing the detection efficiency of alpha and low energy beta particles by the detector.
These and other objects and advantages of the present invention will become apparent upon a consideration of the drawings referred to hereinafter and a complete description thereof.